The utility of telephonic handsets, such as cellular terminals and cordless telephones, in noisy environments is limited by the interfering noise that is passed to the user's ear. To improve the intelligibility of arriving far-end speech in such environments, handsets of the prior art have incorporated such expedients as a volume control to increase the incoming sound signal level relative to the noise signal level.
Another expedient is active cancellation of the ambient acoustic noise pressure relative to the incoming speech acoustic pressure within the user's ear. One approach to active noise cancellation is described, for example, in U.S. Pat. No. 5,491,747, issued on Feb. 13, 1996 to C. S. Bartlett et al. under the title “Noise-Cancelling Telephone Handset”, and commonly assigned herewith.
In typical applications of active noise cancellation, a microphone picks up the ambient noise pressure and generate a signal that is fed into a noise canceling circuit. This circuit creates a noise inverted signal that is applied to the handset receiver. (In this context, the “receiver” is a loudspeaker or other electric-to-acoustic transducer for projecting the received audio signal into the user's ear.) The receiver acoustic output subtractively interferes with the ambient noise pressure, thus reducing the noise level in the user's ear.
It is well known that active noise canceling techniques may be either of a negative feedback design or a feed-forward design. Both of these approaches are described, for example, in P. A. Nelson and S. J. Elliot, Active Control of Sound, Academic Press, 1992. Although the viability of feed-forward designs has been recognized, negative feedback designs have generally been preferred for use in telephonic equipment, such as in headset earpieces. Such a preference is due, in part, to the greater robustness that negative-feedback designs tend to exhibit against inter-user variability. This preference is also due, in part, to the relative ease with which these designs may be implemented in analog circuitry, and to a general perception that feed-forward designs provide an inferior level of noise cancellation. An illustrative negative feedback system of the prior art is shown in FIG. 1.
There has also been a general perception that a feed-forward design can be made robust against inter-user variability only by incorporating adaptive circuitry. However, as a practical matter, such an expedient would call for a digital signal processor (DSP) having two analog-to-digital converters (ADCs)—one each for the reference microphone and the error microphone, respectively, and one digital to analog converter (DAC) to generate the canceling noise signal for the handset receiver. Although recent digital cellular terminals do in fact include a DSP, the requisite number of ADCs is not generally present. Additionally, the computational capacity of the terminal DSP is substantially taken up by the other voice processing functions required by the terminal. Thus, very little computational capacity is left over for implementation of an active noise canceling function. Although there are commercially available some DSPs that have been designed specifically for active noise cancellation, the computational capacity of even these devices is limited as a result of pressure to keep the cost within bounds of commercial feasibility.
Despite their reputed advantages, negative feedback noise canceling designs suffer from certain disadvantages as well. For example, to avoid a potential instability, it is generally desirable to set the feedback gain to a level that is lower than optimum, leading to some performance degradation.
This and other disadvantages could be overcome by a computationally efficient feed-forward noise cancellation design suitable for implementation on a DSP.